Gases which contain sulfurous products such as sulfur oxide and hydrogen sulfide, flue gas or foul gas, pose a major environmental problem in that the sulfur containing compounds can form acid rain under proper atmospheric conditions and therefore must be removed from the gases before they can be discharged into the atmosphere. Such gases are produced by the combustion of various fuels and as waste gases from petroleum cracking processes and the like. Substantial research has been conducted in the so-called wet sulfurization technology which is attempting to develop processes suitable for the treatment of large volumes of foul gases and gases which contain high concentrations of sulfur oxides and hydrogen sulfide. Thus sulfur oxides are removed from flue gases and the like by contacting the sulfur oxide containing gas with a water solution of sodium sulfite to produce acid sodium sulfite. The acid sodium sulfite is then treated with calcium carbonate or calcium hydroxide to generate calcium sulfite crystals which are subsequently oxidized to gypsum. A similar method involves a slurry of calcium hydroxide which is then contacted with the flue gas to produce calcium sulfite which is subsequently oxidized to gypsum. In yet another method flue gas is contacted with an aqueous slurry of magnesium oxide to form magnesium sulfite which may then be thermally decomposed to regenerate SO.sub.2 and MgO.
Gases containing hydrogen sulfide may be subjected to the well known amine absorption or Klaus process. Another process for the treatment for gases containing H.sub.2 S is the Hiperion process, for example as described in U.S. Pat. Nos. 3,459,495, 3,937,795 and 4,255,400, is a single step liquid redox hydrogen sulfide removal process that uses a naphthoquinone chelate catalyst. Another process for the treatment of gas to remove SO.sub.2 is referred to as the Takahax process and is defined in U.S. Pat. No. 3,459,495. Hydrogen sulfide may also be extracted from a gas stream by absorption in an aqueous solution of a catalytic reagent containing vanadium compound or an iron chelate.
The sulfur oxide removal processes, although effective for the removal of sulfur oxides from a gas, present substantial reaction product disposal problems as well as exhibiting low efficiency which require the use of high quantities of reactants. The processes for H.sub.2 S removal also involve the use of substantial quantities of the catalyst for converting H.sub.2 S to elemental sulfur. The elemental sulfur recovered from aqueous catalytic processes contains a sufficient amount of catalyst material to reduce the commercial value of the sulfur thus produced.
The elemental sulfur produced in the foregoing processes for stripping H.sub.2 S can be recovered and purified by heating to a temperature above the melting point of the sulfur but below the boiling point of the catalyst or stripping solvent, or by maintaining a stripping solvent at a relatively high temperature and subsequently cooling the stripping solvent to cause the precipitation of the sulfur therefrom.
In U.S. Pat. No. 5,049,370 there is described an environmentally benign system for recovering elemental sulfur from an aqueous cake by contacting the cake with an organic solvent at an elevated temperature and permitting the mixture to separate into an organic phase in which the sulfur is solubilized while leaving the catalyst residue in the aqueous phase. A number of electrophilic solvents are disclosed as being suitable for solubilizing substantial quantities of sulfur including such water immiscible solvents as xylene, ethyl benzene, tetra hydronaphthalene, petroleum naphtha, trichloroethylene, carbon tetrachloride and the like. Other electrophilic compounds include amines, alcohols, glycols, aromatic hydrocarbons which may be used alone or in combination with a base solvent which is water immiscible.
As mentioned above, however, the processes for stripping H.sub.2 S require substantial quantities of catalyst which imposes an economic burden of substantial magnitude on the process. The table illustrates the catalyst consumption in tons per month for typical high volume H.sub.2 S stripping processes over various types of foul gas.
TABLE A ______________________________________ CATALYST USE GAS as napthaquinone VOL- H.sub.2 S t/month TYPE OF UME CON- Takahax Hiperion FOUL GAS Nm.sup.3 /hr TENT Process Process ______________________________________ Coke Oven Gas 100,000 10 g/m.sup.2 5.36 3.22 Petroleum Cracked 100,000 13% 105.85 63.50 Gas Petroleum Hydroge- 100,000 99% 80.61 48.36 nation Gas Methanol Fermenta- 150,000 3% 36.39 21.98 tion Gas ______________________________________
Accordingly, it would be highly desirable to provide a process for the desulfurization of sulfur containing gases which can handle large volumes of gas utilizing a minimum of catalyst material thereby to reduce the economic burden of desulfurizing gases.